Link Layer Protocol and Error Correction

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Questions and Answers

What is the purpose of the 'adapter' in the link layer?

To perform error correction

To implement the link layer protocol (correct)

To assign IP addresses

To route datagrams

What is the function of the sending side in the link layer?

To perform error correction

To route datagrams to their destination

To extract datagrams and pass them to the receiving node

To encapsulate datagrams in a frame and add error checking bits (correct)

What is the goal of the Internet checksum?

To route datagrams to their destination

To detect errors in the transport layer (correct)

To perform error correction

To assign IP addresses

What is the role of the receiving side in the link layer?

To extract datagrams and pass them to the receiving node Signup and view all the answers

What is the purpose of error detection and correction bits (EDC)?

To detect and correct errors Signup and view all the answers

What is the limitation of error detection?

It may miss some errors, but rarely Signup and view all the answers

What is the primary purpose of a LAN address?

To get a datagram from one interface to another physically-connected interface Signup and view all the answers

How are MAC addresses allocated?

By the IEEE, which administers the MAC address space Signup and view all the answers

What is the size of a MAC address in most LANs?

48 bits Signup and view all the answers

What is the analogy for a MAC address?

Like a Social Security Number Signup and view all the answers

What is the purpose of ARP?

To translate IP addresses to MAC addresses Signup and view all the answers

What is the size of an IP address?

32 bits Signup and view all the answers

What is the primary function of the ARP protocol?

To resolve IP addresses to MAC addresses Signup and view all the answers

What is the purpose of the TTL in an ARP table?

To specify the time after which an ARP entry will be forgotten Signup and view all the answers

How does a device on a LAN typically obtain the MAC address of another device on the same LAN?

By broadcasting an ARP query packet Signup and view all the answers

What is the main purpose of the preamble in an Ethernet frame?

To synchronize the sender and receiver clock rates Signup and view all the answers

What happens when a device on a LAN receives an Ethernet frame with a destination address that does not match its own MAC address?

The device discards the frame Signup and view all the answers

What is the term for information that times out or goes away unless it is refreshed?

Soft state Signup and view all the answers

Study Notes

Link Layer and ARP

A wants to send a datagram to B, knowing B's IP address, but needs B's MAC address
ARP (Address Resolution Protocol) is used to determine MAC address of B
Each IP node (Host, Router) on LAN has an ARP table, which maps IP addresses to MAC addresses
ARP table entries have a Time To Live (TTL) of typically 20 minutes

ARP Protocol

A sends an ARP query packet with B's IP address, which is broadcast to all machines on the LAN
B receives the ARP packet and replies to A with its MAC address
A caches the IP-to-MAC address pair in its ARP table until the information times out
ARP is "plug-and-play" and nodes create their ARP tables without intervention from the network administrator

Ethernet Frame Structure

Preamble: 7 bytes with a pattern of 10101010 followed by one byte with a pattern of 10101011
Addresses: 6 bytes (destination MAC address)
Type: indicates the higher layer protocol (mostly IP, but others may be supported)
CRC: checked at the receiver, and if an error is detected, the frame is dropped

Adapters and Link Layer Protocol

Link layer is implemented in the "adapter" (aka NIC or Ethernet card)
Sending side: encapsulates the datagram in a frame, adds error checking bits, and more
Receiving side: looks for errors, extracts the datagram, and passes it to the receiving node

Error Detection and Correction

Error Detection and Correction (EDC) bits are added to the data for error detection
Error detection is not 100% reliable, and larger EDC fields yield better detection and correction
Internet checksum: detects "errors" (e.g., flipped bits) in transmitted segments at the transport layer
Sender: computes checksum, and receiver verifies it to detect errors

LAN Technologies and Addresses

LAN technologies: Ethernet, hubs, bridges, switches, 802.11, PPP, ATM
LAN (or MAC or physical or Ethernet) address: used to get a datagram from one interface to another physically-connected interface
48-bit MAC address is burned in the adapter ROM, and allocation is administered by IEEE

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Description

This quiz covers the basics of the link layer protocol, including error correction, half-duplex and full-duplex communication, and the role of adapters and frames in data transmission.